In the Darwin Centre at London’s Natural History Museum, Billy Harrow, a cephalopod specialist, is conducting a tour whose climax is meant to be the Centre’s prize specimen the Giant Squid. But Billy’s tour takes an unexpected turn when the squid suddenly and impossibly vanishes into thin air. As Billy soon discovers, this is the precipitating act in a struggle to the death between mysterious but powerful forces in a London whose existence he has been blissfully ignorant of until now, a city whose denizens --- human and otherwise --- are adept in magic and murder.

When a murdered woman is found in the city of Beszel, somewhere at the edge of Europe, it looks to be a routine case for Inspector Tyador Borlú of the Extreme Crime Squad. To investigate, Borlú must travel from Beszel to its neighbor, the city of Ul Qoma. But this is a border crossing like no other, a journey as psychic as it is physical, a seeing of the unseen. As the detectives uncover the dead woman’s secrets, they begin to suspect a truth that could cost them more than their lives.